Metal-Poor Stars. III. on the Evolution of Horizontal-Branch Stars

We describe the evolution of low-mass stars of composition F = 0.1, 0.2, 0.3 and log Z = -3,4 during the phase of core helium burning. Several models for F = 0.3 are evolved until a large carbonoxygen core develops. By comparing properties of giant-branch and horizontal-branch models with the observations, we find that F 0.29 + 0.05 best describes cluster stars if Z = 10- A0- . For such compositions, model tracks during core helium burning are all quite short compared with the color width of horizontal branches appearing in nature. We conclude that there may be a spread in stellar mass along the horizontal branch that is of the order of 0.1-0.2 M0. We show that if the mass distribution is continuous, the horizontal branch will define a solid wedge in the color-magnitude diagram, regardless of whether or not an individual track exhibits a blueward hook. Using current estimates of the initial mass of the helium core when losses due to plasma neutrinos are included, we estimate ages for M3, M92, and M15 to be on the order of 11-13 billion years. RR Lyrae luminosities are consistent with those given by the method of statistical parallax and are only slightly larger than those currently suggested by pulsation theory when fitted to the observations. We infer that most of an observed rate of period change dP/dt for RR Lyrae stars is "noise." The distinct bias toward positive values of dP/dt in ", Cen and the small value of (dP7dt) in M3 can be interpreted as evidence that stars in M3 pass through the instability strip during the major phase of core helium burning and hence have high Z, whereas stars in c Cen pass through the strip rapidly from blue to red toward the end of core helium burning and hence have low Z. Finally, we identify the secondary or asymptotic giant branch in globular clusters as the residence of stars that are burning helium in a shell outside a core. These stars evolve upward and to the red in the H-R diagram.